Figure out the right approach when a updateUI is called on a BackgroundFetchSuccessEvent instance they've created themselves.

Currently, we create a promise that will never settle.
We could eagerly react and throw.

Assigning to Peter per review comment here,
https://chromium-review.googlesource.com/c/chromium/src/+/1158835/10/third_party/blink/renderer/modules/background_fetch/background_fetch_update_ui_event.cc#49